Building materials for railway sleepers intended for operation in the arctic zone

. The article shows a picture of climatic changes in the Arctic zone, presents the advantages of reinforced concrete sleepers, argues the necessity of using in the Arctic zone sleeper materials with high strength and frost resistance, high deformation and corrosion resistance, gives a brief historical overview of the contribution of St. Petersburg University of Railway Transport to the development of highly effective materials for the production of critical structural elements of the upper structure of the railway Data on the change in the strength and deformation characteristics of slag-alkali concrete for reinforced concrete trusses after 10 years of operation are presented. The paper assesses the state of the reinforcement and the factors influencing the possible corrosion development: dynamics of alkali binding, composition of passivating films and the adjacent layer of concrete, microstructural analysis of the reinforcement.


Introduction
The Arctic is a single physical and geographical region of the Earth, adjacent to the North Pole and including the outskirts of the continents of Eurasia and North America, almost all of the Arctic Ocean with islands (except the coastal islands of Norway), as well as the adjacent parts of the Atlantic and Pacific Oceans. The southern border of the Arctic coincides with the southern border of the tundra zone. Its area is about 27 million km² [1].
The territory of the Arctic includes a vast drifting ice shelf in the Arctic Ocean, the northern waters of the two oceans, the Pacific and Atlantic, islands and archipelagos including Greenland, the polar lands of North America and Eurasia, and many seas [2]. The official representative states on the shelf are: Russia, Canada, Denmark, Norway, USA. The Russian Federation includes a fairly large part of the Arctic [1].
Arctic territories of the Russian Federation: The land territories of the Arctic Zone of the Russian Federation are defined according to Presidential Decree No. 296 of May 2, 2014 "On the land territories of the Arctic Zone of the Russian Federation." These include: • Murmansk Region (in full).
thawing of frozen strata in the cryolithic zone occurs, due to the significant content of ice in them, the average subsidence of soils may be 10 meters or more. Clear evidence of a warming climate is the thawing of the perennial, so-called permafrost. In a large part of Russia's cryolithozone, which occupies more than 60% of the country (it is the largest permafrost massif in the world under a single national jurisdiction), over the period from the 1970s to the 1990s a trend towards higher temperatures in the upper layers of permafrost associated with atmospheric warming was clearly pronounced. Although climatic changes in the European part of Russia are weaker than in Siberia, changes in the state of the permafrost here are no less significant. Over the past 20-30 years, the temperature of the permafrost in the European part of the Russian Arctic and Subarctic has increased from +0.22 to +1.56°C, and the number and thickness of thalics (underground thaw areas) has increased. Observational data also show a progressive increase in the seasonally thawed permafrost layer and in the value of ground subsidence over the last 10 years in some regions of the Russian Arctic by 14-80% (Kolyma lowland, Eastern Chukotka, Bolshezemelskaya tundra) [8].
The operating conditions of track structure elements in the special conditions of the Arctic zone impose higher requirements on the materials of their structures.
In light of the above, the question arises about the stability of preserving the monolithicity (integrity) of railroad track structures under changing climatic conditions in the Arctic zone.
The design and selection of railroad track structure should be based on the following basic provisions developed by Professor G.M. Shakhunyants [9].
1. The track is a single structure in which all elements work together. Changes in the design or operation of at least one element cause changes in the operation of each of them and the track as a whole, in the interaction of the track and rolling stock, in the costs of maintenance and repair of track and rolling stock (in part, depending on the track).
2. Design of a track, work of a track as a whole and each its element, terms of service and expenses for maintenance and repair of a track are defined by volume and conditions of operational work (load intensity of a line, weight and speed of movement of trains, designs of the crew, loadings from wheel pairs on rails, a type of traction, kinds of means of intercourse, character of transported cargo, etc.). Local peculiarities (climatic, geological, etc.) play a significant role. 4. Economic feasibility and national interests and objectives determine the final choice of track design and its elements, the system of organization and mechanization of work on the construction, maintenance, repair and strengthening of the track. It is extremely important to ensure optimum track elasticity throughout its entire length. And this can only be achieved by the competent design of each element of the track, ensuring the solidary work of all elements for their entire service life.
Railroad track is exposed to the following influences: rolling stock, with the impact of the locomotive determining the strength of the track, and the cars (as mass loads) -the residual deformations; natural and climatic factors, of which the main ones are temperature and precipitation; intrinsic (internal) stresses occurring in track structure elements, mainly in rails, during their manufacturing, laying, and operation.
Correct selection, sorting and laying of track structure materials during construction and repairs can significantly increase track uniformity and reduce the level of dynamic interactions of rolling stock and track, which will improve track condition, reduce labor intensity during routine maintenance and increase the periods between its repairs.
These requirements affect the choice of design track and its components, the organization of laying, maintenance and repair of track and should be considered comprehensively in a single and inseparable connection with each other, taking into account the purpose of the main railroad track -to ensure safe passage of trains with the established speeds and unconditional performance of the required volume of traffic in the most appropriate way.
In view of the diversity of the freight load, from a very insignificant load on local branches to the load of the main lines exceeding 100 million tons of gross load per 1 km of a single track per year and the great variety of climatic and geological conditions, it is obvious that different types of tracks, their superstructures and subgrade must exist. The task is to apply the best types for each specific case. For this in modern conditions of rapid technological progress requires a continuous improvement and improvement of existing types and the search for new ones, the use of modern materials, the development of the most appropriate design solutions and the discovery of new opportunities.
A sleeper is one of the main structural elements of the railroad track superstructure. Sleepers are used for: -absorbing pressure from the rails and transferring it to the ballast layer; -elastic processing of dynamic effects on the track; -ensuring track gauge constancy and, together with ballast, the stability of the rail-tie grid in the horizontal and vertical planes.
In accordance with this, sleepers should have sufficient strength, elasticity, good resistance to mechanical wear and movement, be simple in form, have the longest service life and the lowest cost in manufacturing and maintenance.
At the earliest stage of railroad development, wood was the dominant material used in sleeper production. During the 20th century, new materials began to be used in response to the need to absorb higher axial loads and high speeds. Today there are the following types of railway sleepers: 1. Wooden sleepers 2. Concrete sleepers 3. Metal sleepers 4. Sleepers made of synthetic materials 5. Composite sleepers In Europe and Asia, with numerous high-speed lines and heavy traffic, the use of prestressed concrete sleepers, which are more durable and require less maintenance, has become the norm. In the rest of the world, however, the use of wood is still very widespread, such as in the United States, where in 2008 91.5% of sleepers in use were still wood. In the case of Spain, at the beginning of the 21st century, only 8% of sleepers were made of wood, with values in the order of 25% in countries like France or Germany [10].
Advantages of reinforced concrete sleepers: • long service life (from 40 to 50 years); • do not succumb to corrosion; • rail stability; • uniformity of railways Disadvantages of reinforced concrete sleepers: • increased electrical conductivity; • sensitivity to shocks (in particular at the joints); • track rigidity, which leads to faster wear and tear of rails at the joints; • significant weight of sleeper (on average, 270 kg/pc); • the complexity of installation; • susceptibility to temperature fluctuations; • high cost. In connection with the available information on climatic changes and the resulting thawing and subsequent deformation of soils and foreseeable salt aggression, higher requirements for strength and deformation characteristics should be imposed on the sleeper materials as an element of the overhead structure.
We should not forget about the positive properties of wooden sleepers, especially in difficult climatic conditions of the territories of the Arctic zone, which include: • fast payback, they are much cheaper than reinforced concrete sleepers; • the weight of a wooden sleeper (on average 65 kg) is three times less than the weight of a reinforced concrete sleeper; • ease of processing; • easy attachment of rails; • the ability to smoothly divert rail track widening from curves with small radii (less than 300 m); • good adhesion to crushed stone; • possibility to increase or decrease track gauge; • resistance to temperature variations; • dielectric properties of wood; • resistance to adhesion to the substrate; • elasticity. However, we should also not forget about the disadvantages of wooden sleepers Disadvantages: • relatively short service life, the highest quality wooden sleepers currently have a service life of no more than 18 years; • consumption of scarce structural wood. The main problem of wooden sleepers is their tendency to rot in the places where rails are attached to them, which is unlikely in Arctic conditions, and the problem with their further utilization in case of impregnation.
Wooden sleepers have no restrictions on laying zones, it is advisable to lay them first of all: − of the link track, where it is necessary to widen the track gauge to 1530 -1535 mm; − frost-prone; − highly stressed, where the use of continuous track with reinforced concrete sleepers is ineffective [11].
Back in 1994, the regulation on the track management system on the railroads of the Russian Federation noted that the track should be on reinforced concrete sleepers, and the track to be used as a link track on wooden sleepers. These requirements are more stringently reflected in the 2001 regulation, which, in particular, emphasized: the continuous track should be on reinforced concrete sleepers, and "the use of track links on wooden sleepers is coordinated with the Department of Track and Structures of the Ministry of Railways" [12].
Department of "Building Materials", created in the walls of the oldest transport university in our country in 2020 celebrated its 100th anniversary. Currently, the department is called "Construction materials and technologies".
It should be noted the enormous contribution of scientists of the department to the development of science of concretes for the needs of railway construction.
In 1950-1951 were begun researches in the field of new silicate materials in development of first works of professor V.P.Nekrasov -the author of term "silicate". In contrast to the proposal of V. P. Nekrasov and the Estonian scientist I. A. Hint, the technology developed by the department was based on the creation of a new binder -lime-quartz cement of autoclave hardening based on a fine grinding. This work was carried out under the leadership of Professor V.P. Petrov and Professor A.V. Satalkin. In the joint work on this scientific direction participated Department of "Railway track". Research of compositions of silicate concretes for railway sleepers, their structure and production technology was carried out in the laboratory of the department and in production conditions at Pavlovskij factory of silicate bricks, Lodeynopolskij factory of building products, Mginskij factory of concrete products and Kiev factory of concrete sleepers. Pilot batches of factory-made sleepers and concretes were tested for dynamic and static loads, deformability and resistance to the action of stray currents, frost resistance, and bonding of reinforcement with concrete. High-strength concrete compositions and production technology were developed, new design solutions of sleeper forms and accessories for their production were proposed, temporary technical specifications were issued, the condition of sleepers during and after their long-term operation was studied.
New cement was used for preparation of high-strength (up to 80-90 MPa) mortars and concretes used in transport construction. At the same time the water resistance and frost resistance of concrete increased. By 1959, factory technology for production of railway tension-reinforced sleepers was developed. Pilot batches of «ШАС 1» and «ШАС 2» type sleepers made of silicate concrete in 1958-1961 were laid in the main track on the sections of the Oktyabrskaya railroad.
Research conducted at the department in 60-70s showed the effectiveness of using finegrained cement-sand concrete with additives for production of railway sleepers. The influence of water-soluble resins on the deformative properties of concrete was studied, the effectiveness of some polymer coatings on concrete was shown to increase the durability of reinforced concrete centrifugal supports of contact network, used on the railway transport In the 70's, the Department began to develop directions related to the use of ashes, slags, microsilica, etc. in the production of building materials. The direction began to be developed (works of prof. O.V. Kuntsevich) for the production of new types of binder -low water demand binder (LWB) and on its basis -obtaining various types of high-strength and highfreeze resistant concretes.
Since 1984, Associate Professor T.M. Petrova begins to develop a new scientific direction -uncemented high-strength concretes on the basis of slag-alkaline binders. Together with the Department of "Railway track" in 1989-1991 the first time in the country were made in the factory of concrete sleepers in Chudovo reinforced concrete sleepers and tie beams with slagalkali binders. After successful static and dynamic tests in laboratory conditions by the team of scientists of the department "Construction Materials" of PSURT, headed by the future doctor of technical sciences, professor, who was the head of the department since 1999 T. M.
Petrova, in 1990 the first track-switch on bar sections from wood-alkali binder was installed on the October railroad on the St. -Petersburg -Moscow line. The choice of this type of structures was due to the fact that translating beams are used in more difficult conditions than sleepers on the line, and they have variable length up to 5 meters. The service life of transferable bars can be 1.5-2.0 times shorter than that of sleepers [13,14].

Materials and methods
In the manufacture of the structures, ground granulated blast-furnace slag from the Cherepovets metallurgical plant with basicity modulus Mo=1.04 was used; the alkaline component was soluble sodium silicate with Ms=1.5. Steel, prestressed, periodic profile wire of Bp class with a diameter of 3 mm was used as reinforcement in the railway bars. The samples were tested at the age of 28 days after manufacture; under the conditions of laboratory storage at the age of 10 years; after operation on the track at the age of 10 years. To study the corrosion resistance of the bar material was carried out on an electron scanning microscope Akashi, ABT-55, Japan.

Results and analysis of results
After the long-term operation of the track crossing beams made of slag-alkali concrete on the road (for 10 years), there was a unique opportunity to investigate the changes that occurred in the material of the structures.
The results of strength characteristics of the samples taken from the structures and the samples of vintage age are given in Tab. 1 The results of the research showed that there were no significant changes in both compressive and tensile strength characteristics. This indicates that concrete, as a result of long-term operational influences, has not undergone significant destructive changes. The deformative properties of concrete have a great influence on the quality and durability of concrete and reinforced concrete structures. Table 2 shows values of elastic moduli and corresponding relative deformations of concretes (stresses were 0,3 Rb). The obtained data testify to the fact that during the work of structures in the conditions of railroad track, the modulus of elasticity of slag-alkali concrete has lower values than that of Portland cement concrete, which predetermines high deformability and better resistance of the former to the influence of dynamic loads during the work of subrail foundations. High deformability of slag-alkali concrete will have a positive impact not only on the durability of subrail base structures, but also on the increase of the service life of rolling stock.
Corrosion resistance of reinforcement is one of the most important factors that influence the durability of the whole reinforced concrete structure. Especially actual are the questions of corrosion of reinforcement in constructions, working in the way, under the influence of dynamic and static loads, specific aggressive factors, alternating moistening and drying, freezing and thawing, stray currents.
The study of properties of slag-alkali concrete samples exposed to prolonged exposure to direct current and their comparison with properties of portland-cement-based concrete have shown that pH of slag-alkali concrete samples at mixing with soluble glass with MС=1.5 makes 12.2-12.7, at portland-cement samples during the same period of time decreases to 7.8-8.4.
The results of evaluation of corrosion resistance of prestressed reinforcement are given below as an answer to the frequently arising question about its condition during long-term operation.
Samples drilled out of structures after 30 years of operation in the track were subjected to tests.
It is known that corrosion of reinforcement in reinforced concrete structures is one of the causes of their damage. The corrosion resistance of steel reinforcement of concrete is affected by many factors, of which the structure and composition of concrete, the thickness of the protective layer, climatic conditions, etc. have the greatest importance. The process of corrosion of metals in the vast majority of cases has an electrochemical mechanism. Due to the electromechanical heterogeneity of the metal surface, micro-galvanic elements are formed on it in contact with the electrolyte. At the anode sites the metal dissolves by the Me→Men++ne scheme (anodic oxidation), the ionization of atoms transferring in the form of hydrated anions into the solution. The resulting free electrons, remaining in the crystal lattice of the metal, move to the cathodic parts of its surface. An electrical circuit closes through the electrolyte solution. In addition to the anodic and cathodic reactions, which are the main reactions, secondary reactions leading to the formation of corrosion products can have a great influence on the rate of corrosion. If corrosion products are well soluble, they can be transported away from the site of their formation. Hardly soluble, which are usually the corrosion products of steel -are deposited on its surface, forming an increasing layer of rust consisting of iron oxides and hydroxides. As a rule, these reactions run in parallel.
Our research allowed us to estimate the dynamics of alkali binding in slag-alkali concrete for the period from 1988 to 2018 (Table 3). Analysis of the table shows that the main amount of free alkali passes into hardly soluble compounds during the first year. Over the subsequent period of time, a gradual increase in the binding of the alkaline component up to the age of 30 years was noted.
During our visual inspection of the bars removed from the track after many years of operation, no visible traces of corrosion were found.
Further analysis was carried out on samples -cores drilled from these structures in the most characteristic points.
It is known that one of the main features of long-term preservation of reinforcement in concrete is formation of protective films on its surface having thickness from 0.002 to 0.010 µm, according to some authors, and up to 0.005 µm in the absence of calcium ferrites, according to others.
In the course of the research, element-by-element analysis of the surface of the reinforcement after the structures have been in transit has been carried out. It has been noted that not only calcium and silicon, but also sodium take part in the formation of the passivating film of slag-alkali stone, while in the subsequent layer of concrete -at a distance from the reinforcement, its amount decreases slightly and is Na: 11,909-16,317 %, with a decrease in Fe content -4,777-2,694 %, respectively. X-ray phase analysis confirmed that there are no corrosion product minerals in the layer adjacent to the reinforcement. Table 5 and Fig. 1 show the results of estimation of element-by-element composition of slag-alkali stone at a distance from the reinforcement after the structures have been in transit.  To study the change in cracking in slag-alkali concrete after long-term operation, samples were taken from the structure and electron images were taken from them (Akashi electron scanning microscope, ABT-55, Japan).
The structure of the concrete after long-term operation of the structure in the track is shown in the figure. Analysis of the obtained results showed that the system of microcracks of slag-alkali stone did not undergo fundamental changes during operation. The system of microcracks remained discrete, the width of microcrack opening slightly increased. If the average width of microcracks in unstressed stone stabilized within 0.1 -1 μm, the width of microcrack opening in the studied samples was 1 -2 μm (maximum value -4 μm).
Taking into account such an insignificant discrepancy and the time that the structure was in transit, it can be assumed that the established system of microcracks is stable.
Electronic images of the slag-alkali structure at different distances from the reinforcement allow us to confirm the absence of corrosion damage to the reinforcement and destruction of the layer of concrete adjacent to the reinforcement.
The conducted researches allowed to draw a conclusion about high corrosion resistance of steel reinforcement of sub-rail base structures made of prestressed slag-alkali reinforced concrete, in severe operating conditions of railroad track over the period of 30 years [15÷19].
Even a brief historical excursus shows the great contribution of scientists of the department "Construction Materials and Technologies" in the design of concrete of high strength and frost resistance and production of reinforced concrete sleepers, which shows the great potential of scientific achievements of scientists of the department in the study of materials for railway sleepers, designed for operation in the Arctic zone conditions. Scientific achievements of scientists of the department "Construction materials and technologies" undoubtedly indicate the great potential of the obtained results for highstrength, corrosion-resistant concretes with high frost resistance for the Arctic region with low temperatures.
Use of slag-alkali concretes, frost resistance of which can reach F1300, has good prospects for sleeper materials in construction of railroads in climatic conditions of the Arctic zone. High endurance of structures made of slag-alkali concrete under the joint action of dynamic loading and natural factors has been revealed, as well as high corrosion resistance of slag-alkali concrete in organic-oil environment, typical for service of subrail structures in the track and high corrosion resistance of steel reinforcement of constructions.
However, it should be noted that in conditions of actual and predictable climatic changes, accompanied by thawing with subsequent deformation of soils, scientists are faced with new tasks to find more perfect materials of reinforced concrete sleepers, having along with high strength, corrosion, deformation properties, high frost resistance, adapted to changing climatic conditions.